1. Field of the Invention
This invention relates to a line exposure type image forming apparatus including a micromirror device having numerous micromirrors arranged in a matrix of rows and columns, each micromirror being tiltable between an exposing posture for reflecting light incident from a light source to a predetermined exposure position on a photosensitive material and a non-exposing posture for reflecting light to a location other than the photosensitive material, a sub-scan moving mechanism for moving the photosensitive material relative to the exposure position, and a mirror controller for controlling the postures of the micromirrors in response to image signals.
2. Description of the related Art
The micromirror device is also called a DMD (Digital Micromirror Device) which actuates, by action of electrostatic fields, square micromirrors of high reflectance formed on a wafer by aluminum sputtering, each with a side measuring approximately 16 (m. Several hundreds of thousands to a million of these micromirrors are squeezed into a matrix form on a silicon memory chip by CMOS semiconductor technique. Each micromirror can swing about a diagonal to tilt to two steady postures about (10 degrees relative to the horizontal. This micromirror device usually is employed, with each micromirror corresponding to one pixel, in a digital display system for projecting digital images on a screen by controlling a reflection time of light from a light source.
The micromirror device may be used to form exposure dots in selected positions on a photosensitive material, with each micromirror, in one of the two steady postures, reflecting the light from the light source to a predetermined position on the photosensitive material, and in the other posture, reflecting the light outside the region of the photosensitive material. Further, the exposure dots may be given a desired gradient by controlling irradiation time when producing the exposure dots. The micromirror device may thus be employed also in a line exposure type image forming apparatus which forms images on a photosensitive material by moving the photosensitive material relative to the irradiating positions of the micromirrors. Such image forming apparatus are known from U.S. Pat. No. 5,953,103, U.S. Pat. No. 5,933,183 and Japanese Patent Application Laying-Open Publication S10-104753, for example.
However, each of the above conventional apparatus makes use of a micromirror device designed for a digital display system. This micromirror device is not necessarily optimal for a line exposure type image forming apparatus. For example, the resolution of exposure dot images formed on a photosensitive material is dependent on the degree of mirror integration of the micromirror device. A micromirror device enabling an XGA screen resolution has 768 pixels vertically and 1024 pixels horizontally. Even where the rows, which are the longer, are used as lines to effect line exposure, and where photographic paper of 100 mm in width is used as a photosensitive material, the resolution will be about 250 dpi. Thus, demands for a higher resolution cannot be met. It is possible to double the resolution by using two identical micromirror devices, but this will entail an excessive increase in cost.
The object of this invention is to provide a line exposure type image forming apparatus employing a micromirror device, which realizes improved resolution while avoiding an increase in cost.
The above object is fulfilled, according to this invention, by a line exposure type image forming apparatus having a micromirror device disposed such that an imaginary line linking imaging positions on the photosensitive material of the micromirrors in a predetermined line is at an angle to a direction of relative movement of the photosensitive material, and an exposure dot line is produced on the photosensitive material in a main scanning direction perpendicular to the direction of relative movement by a main scanning mirror set formed of micromirrors selected in a direction at an angle to a direction of the columns of the micromirror device.
In this construction, the rectangular micromirror device is set to an oblique posture to utilize, as an exposure line, micromirrors disposed along an oblique line, instead of micromirrors in a vertical line (direction of rows) or micromirrors in a horizontal line (direction of columns). This allows use of a larger number of micromirrors as the exposure line than where micromirrors in a vertical or horizontal line are used.
To allocate a maximum number of micromirrors to the exposure line, use may be made of micromirrors disposed along a diagonal of the rectangular micromirror device, for example. As a result, numerous micromirrors arranged around the diagonal are in a pixel shifting positional relationship. An appropriate selection of micromirrors to be used in time of printing will allow for a remarkable improvement in resolution.
Where the above-mentioned oblique line acting as basis for selecting micromirrors to be used as the exposure line is inclined at 45 degrees, light beams reflected by the micromirrors conveniently have a uniformed overlapping of luminance distributions. Thus, in a preferred embodiment of this invention, where the micromirror device is rectangular, i.e. where the micromirrors are arranged in a matrix of m rows and n columns, m being smaller than n, the main scanning mirror set is formed of a plurality of subsets each having a predetermined number of micromirrors selected in a direction at an angle of 45 degrees to the direction of the columns of the micromirror device. The exposure dot lines produced by the subsets are shifted 5 from each other in the direction of relative movement of the photosensitive material (usually called the sub-scanning direction). In practice, however, a single straight exposure dot line is formed on the photosensitive material by the technique of controlling timing of driving the micromirrors in the sub-scanning direction. Since the micromirrors in each subset are arranged along a line inclined at 45 degrees, light beams reflected by the micromirrors have a uniformed overlapping of luminance distributions. The scanning mirror set is divided into a plurality of subsets as noted above. These subsets are shifted from one another only by a small amount. Thus, even where the light source has variations in luminance distribution, differences in luminance of incident light beams are negligible.
Where variations in luminance distribution of the light source are immaterial, it is proposed, as another embodiment of this invention in which the micromirror device is rectangular, that the main scanning mirror set is formed of a first subset having a predetermined number of micromirrors selected in a direction extending from one end of a diagonal of the matrix at an angle of 45 degrees relative to the columns of the micromirror device, and a second subset having a predetermined number of micromirrors selected in a direction extending from the other end of the diagonal at an angle of 45 degrees relative to the columns of the micromirror device. In this construction, the number of subsets is only two, which simplifies the timing control of the micromirrors to absorb the shift in the sub-scanning direction between the exposure dot lines produced by the subsets.
In a further preferred embodiment of this invention, at least three main scanning mirror sets are formed to produce exposure dots, each struck by light beams from different micromirrors for color exposure. This construction forms color exposure dots of high quality with color discrepancy restrained, thereby forming a high quality color image. In this case, three or more, preferably a multiple of three, main scanning mirror sets may be formed to produce exposure dots of at least one color, preferably three colors, struck by light beams from different micromirrors for color exposure. Then, the quantity of radiation to form each exposure dot may be distributed to a plurality of micromirrors. This results in an increase in the speed of movement in the sub-scanning direction, i.e. the speed of the exposing process. In a still further preferred embodiment of this invention, the main scanning mirror set, and an interpolative main scanning mirror set formed of micromirrors for producing exposure dots between exposure dots produced by the micromirrors of the main scanning mirror set, produce an exposure dot line of increased resolution. This construction effects so-called pixel shifting only by the technique of controlling timing of driving the micromirrors in the sub-scanning direction. The exposure dots thereby formed have a twofold resolution. Other features and advantages of this invention will be apparent from the following description of the embodiments to be taken with reference to the drawings.